This invention relates to compositions comprising terblends of linear low density polyethylene (LLDPE) resins having increased extrudability, overall processability, and output; said blends provide films of high quality without losing overall film properties.
It is a well known fact that linear low density polyethylene copolymers generally produce films of high quality when compared to high pressure or branched low density polyethylenes (HP-LDPE). LLDPE films exhibit higher tensile strength, outstanding puncture resistance, and enhanced impact strength. However, the extrudability of LLDPE resins is poor, mainly because of high shear viscosity. Due to the absence of long chain branching, the shear viscosity of LLDPE exhibits less shear thinning behavior than branched LDPE. This non-newtonian rheological behavior brings about extrusion difficulties; that is, limits the extrusion head pressure and results in high motor amperage and high torque and prevents increasing the output under a given set of extrusion conditions.
Furthermore, the extrusion problem is frequently accompanied by the appearance of surface irregularities on the produced film. Surface irregularities or, more generally, "melt fracture" occur over a range of shear rate depending on the molecular characteristics of the polymer. These are characterized by closely spaced circumferential ridges along the extrudate when extruded through a capillary die. In a more severe form, it resembles what is generally known as "sharkskin". The onset of melt fracture is believed to occur at or above a critical shear stress, although the concept is yet fully understood.
In the past many attempts were made to alleviate the above-mentioned extrusion problems. For example, preparation of blends of linear low density polyethylenes with branched or high pressure low density polyethylene are known. Employing various kinds of processing aids; such as, low molecular weight hydrocarbons, fatty amides or bis amides, fluoroelastomers and certain silicon polymers, is also known.
The above-recited disadvantages of LLDPEs are substantially overcome by the resin blends of this invention. We have now discovered that improved LLDPE resin compositions are provided by incorporating (1) high MI and (2) high MFR LLDPE resins into low MI linear low density polyethylenes.